[InstCombine] Combine no-wrap sub and icmp w/ constant.
Teach InstCombine the transformation `(icmp P (sub nuw|nsw C2, Y), C) -> (icmp swap(P) Y, C2-C)`
Reviewers: majnemer, apilipenko, sanjoy, spatel, lebedev.ri
Reviewed By: lebedev.ri
Subscribers: dmgreen, lebedev.ri, nikic, hiraditya, JDevlieghere, jfb, jdoerfert, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D59916
llvm-svn: 357674
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index dc26725..7b58928 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2321,6 +2321,16 @@
const APInt &C) {
Value *X = Sub->getOperand(0), *Y = Sub->getOperand(1);
ICmpInst::Predicate Pred = Cmp.getPredicate();
+ const APInt *C2;
+ APInt SubResult;
+
+ // (icmp P (sub nuw|nsw C2, Y), C) -> (icmp swap(P) Y, C2-C)
+ if (match(X, m_APInt(C2)) &&
+ ((Cmp.isUnsigned() && Sub->hasNoUnsignedWrap()) ||
+ (Cmp.isSigned() && Sub->hasNoSignedWrap())) &&
+ !subWithOverflow(SubResult, *C2, C, Cmp.isSigned()))
+ return new ICmpInst(Cmp.getSwappedPredicate(), Y,
+ ConstantInt::get(Y->getType(), SubResult));
// The following transforms are only worth it if the only user of the subtract
// is the icmp.
@@ -2345,7 +2355,6 @@
return new ICmpInst(ICmpInst::ICMP_SLE, X, Y);
}
- const APInt *C2;
if (!match(X, m_APInt(C2)))
return nullptr;